Characterization of the Moloney murine sarcoma virus v-mos protein has been hindered by expression of extremely low levels in transformed cells. In order to analyze the v-mos gene product, a eukaryotic expression system is being developed in which v-mos protein will be produced at elevated levels. Specifically, plasmid vectors have been constructed into which v-mos has been placed under the control of the murine, beta-globin, major promoter in various arrangements. This plasmid DNA containing v-mos and plasmid DNA containing the gene coding for the enzyme, adenine phosphoribosyl transferase (aprt), have been simultaneously introduced into murine erythroleukemia cells lacking aprt (MEL aprt-) using calcium phosphate-mediated DNA coprecipitation or spheroplast fusion. Upon induction of differentiation, it is predicted that the exogenously added betaglobin promoter will be activated to express v-mos protein at elevated levels. This system will provide a method by which to explore the nature of the v-mos protein both in vivo, in these MEL cells, and in vitro, after isolation and purification. In addition, such a powerful eukaryotic expression system would be generally applicable for use in studies of other gene products.